The key region to Republicans' recent success in OH is the "Rust Belt" parts (i.e. northeastern and north-central OH). Trump's protectionism helped win over this region, and both Hillary and Biden were proponents of free trade (which didn't do them any favors here). In 2018, Sherrod Brown's campaign was centered on protecting workers' rights, which allowed him to hold on to more than enough support in this region to win statewide for the third consecutive time.
To expand on this, Trump's entire presidency was spent expanding his base, which makes sense he managed to even flip Mahoning in 2020. A good amount of those voters Trump won over in '16 are in the R column for good, Brown's 2018 numbers in Ohio's industrial strongholds down into Appalachia were a far cry from what he garnered in 2012. If he pulls it off in '24 (too far out to tell how that one goes) he'll probably be the last D to win there for a good bit.
